The invention relates to ignition arrangements for internal combustion engines of the type which operate contact-free, i.e., without the use of mechanical synchronizing switches.
More particularly, the invention relates to ignition arrangements comprised of an electrical fuel-igniting element, such as a spark plug or other equivalent component, connected across the secondary winding of an ignition transformer, with the flow of current through the primary winding of the ignition transformer being alternately permitted to build up and then interrupted, in order to induce voltage surges across the transformer secondary and cause the fuel-ignition element to ignite the combustion mixture in the engine cylinder.
It is known to provide an ignition system of this kind wherein the synchronization of the ignition moment with crankshaft rotation is accomplished by resorting to mechanical switches which are opened and closed in synchronism with crankshaft rotation.
It is also known to provide such systems but without mechanical synchronizing switches, making use instead of inductive means for generating synchronizing signals.
It is known, for example, to make use of an A.C. generator which generates an A.C. voltage waveform. When the generated synchronizing voltage reaches a predetermined level, for instance zero, current flow through the primary of the ignition transformer is interrupted, to create the ignition spark. When the generated synchronizing voltage reaches another value, or else reaches zero again at the end of the same voltage half-cycle, the flow of current through the primary winding is re-established. In theory, therefore, both the times of current establishment and current interruption are crankshaft-synchronized. In practice, however, such synchronizing A.C. voltage generators exhibit speed-dependent performance variations which prevent this mode of operation from actually being achieved. In particular, as a result of rotor reactive effects, eddy current generation and the drawing of load current from the output winding of the synchronizing generator by the control circuitry connected thereto, the zero passage of the generated waveform becomes shifted in such a manner that the duration of 1 voltage half-cycle becomes greater than that of the other voltage half-cycle, and this difference in duration increases with increasing engine speed. As a result, control circuitry synchronized with such a synchronizing A.C. voltage generator, and operative for the purpose mentioned above, will permit current flow through the primary winding of the ignition transformer for a time period, per ignition cycle, which is shorter than the time period during which no current flows through the primary winding. Moreover, with increasing engine speed, the ratio of the conduction time of the primary winding to the non-conduction time of the primary winding decreases with increasing engine speed. As a result, at high engine speeds, the build-up of current flow in the primary winding of the ignition transformer does not proceed to an extent sufficient to produce a satisfactory ignition voltage when the flow of primary winding current is subsequently interrupted.
German Offenlegungsschrift No. 15 39 178 describes an ignition system dependent upon engine speed. In that system, current flow is established in the primary winding of the ignition transformer shortly prior to the ignition moment, and in an engine-speed-dependent manner, and the current is then interrupted at the ignition moment to induce the ignition voltage surge. This prior-art construction makes use of a specially configurated rotor adapted to produce a special output voltage waveshape. The disadvantage of this known construction lies in the fact that the rotor is of relatively complicated form and is accordingly rather expensive to produce. Likewise, the remainder of this prior-art system cannot be used unless a rotor having this special configuration is available. A still further disadvantage of this prior-art system is that the slope of the synchronizing voltage waveform at the region of the zero passage is quite small, the voltage waveform changing only slowly in this region. As a result, there is a substantial region centered about the voltage zero-passage where the voltage waveform has values not too different from each other. Consequently, the ignition can easily be triggered rather substantially in advance of or rather substantially after the synchronizing zero-passage of the synchronizing A.C. voltage waveform.